Apparatus and method for distributed arbitration of shared resources

ABSTRACT

A novel distributed arbitration apparatus in accordance with this invention includes a plurality of electrical devices, such as Industry Standard Architecture (ISA) hub cards, that independently determine availability of a shared resource, such as a bus that interconnects the devices, by use of an arbiter. When a circuit in an electrical device needs to use the bus, the circuit drives a request signal active to an arbiter, for example, in the form of a programmable logic device, such as a Programmable Array Logic (PAL™) device included in the electrical device. The arbiter in turn transmits the request signal to all other arbiters. Therefore, each arbiter receives and monitors request signals from all circuits. If only one request signal is active at a given time, all arbiters receive the active request signal and each arbiter drives an acknowledge signal active to the respective local circuit. The circuit that requested the shared resource then uses the resource. If more than one request signal is active simultaneously, each arbiter determines the occurrence of a collision, and drives a collision signal active to the respective local circuit that then implements a backoff algorithm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of the U.S. patent application Ser. No. 08/399,323, now abandoned, filed Mar. 6, 1995, and entitled "APPARATUS AND METHOD TO UNIQUELY IDENTIFY SIMILARLY CONNECTED ELECTRICAL DEVICES" by Sherman Lee, assigned to the same assignee as the present application and that is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

This invention generally relates to a distributed arbitration apparatus and method in which a number of devices independently determine availability of a shared resource. In particular, this invention relates to repeater cards that independently determine availability of a bus that interconnects the cards.

CROSS-REFERENCE TO TABLE A

Table A of 3 pages, incorporated herein by reference in its entirety, is a listing of the equations of a programmable array logic device included in an ISA hub card that is attached to an identity cable in accordance with this invention.

A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.

BACKGROUND OF THE INVENTION

Ethernet over Unshielded Twisted Pair (UTP) cables based on the 10Base-T standard (as described in sections 13 and 14 of IEEE Standard 802.3i-1990--that is a supplement to ISO/IEC 8802-3:1990, Product Number SH16337 available from IEEE Service Center, 445 Hoes Lane, P.O. Box 1331, Piscataway, N.J., USA 08855-1331, and that is incorporated herein in its entirety), using hub cards (sometimes referred to as "concentrators" or "repeaters") in the center of a star-wired configuration is popular in modern offices. Use of UTP cables that can also be used in telephone systems provides flexibility to moves, changes and additions, without rewiring the modern office.

One example of prior art hub cards are ISA hub cards 101-103 (FIG. 1A) that contain a predetermined number of ports, such as Twisted Pair (TP) ports J4 (FIG. 1A), that support an ethernet with eight 10Base-T stations, for example, personal computers (PCs).

ISA hub cards 101-103 of FIG. 1A are 10Base-T Ethernet repeater cards, such as "ISA-HUB™-KT" available from Advanced Micro Devices, Inc. (AMD™), One AMD Place, P.O. Box 3453, Sunnyvale, Calif., 94086, and described in ISA-HUB™ Users' Manual (also available from AMD™), that is incorporated by reference herein in its entirety.

An ISA hub card, such as card 101, supports the physical topology of an Ethernet network, allowing two or more 10Base-T stations that are connected to connectors J4 to communicate with each other. When data is received on a single port of connector J4, ISA hub card 101 retransmits the received data to all other ports of connector J4, performing signal retiming and amplitude restoration. When data appears simultaneously on more than one port, ISA hub card 101 transmits a collision signal to all ports of connector J4, including ports of the transmitting and receiving 10Base-T stations.

ISA hub cards 101-103 can be supported by a single system board 110 to take advantage of shared costs, for example, of the power suppply. Use of a single system board 110 offers easy expansion by allowing installation of additional hub cards (not shown). Although only three ISA hub cards are illustrated in FIG. 1A, up to eight ISA hub cards can be mounted on system board 110. Use of a single system board 110 also allows use of redundant power supplies, support for multiple LAN segments, support for different media access protocols, such as token ring and different media types such as UTP, coaxial cable and fiber optics.

An expansion interface comprising connectors J1 and J2 (FIG. 1A) in each ISA hub card 101-103 allows a number of ISA hub cards 101-103 to act in concert as a single aggregate ISA hub, thereby providing a single ethernet network for an office that contains a larger number of PCs (e.g. 24 PCs) than the number of ports (e.g. 8 ports) on a single ISA hub card. In such a case, a connector, e.g. connector J1 of an ISA hub card is connected to a corresponding connector, e.g. connector J1, of another ISA hub card. Connection of multiple ISA hub cards is described in for example, "IEEE 802.3 Repeater, Technical Manual" available from AMD™ (above), and that is incorporated by reference herein in its entirety. ISA hub cards 101-103 are interconnected by conventional cables, such as cable 120 (FIG. 1B) that has straight through, e.g. one-to-one wiring 131-135 between a left connector 140 having terminals 141-145 and a right connector 150 having terminals 151-155.

Any of ISA hub cards 101-103 can be configured to perform a master function by placing a single jumper in the "M" position as described in the ISA-HUB™ User's Manual referenced above. All ISA hub cards in an aggregate ISA hub, other than a master ISA hub card, are slaves that are assigned unique identities by placing a single jumper (e.g. jumper JP1 in FIG. 1A) at a unique position other than the M position as described in the ISA-HUB™ User's Manual (above).

In a prior art ethernet, the master ISA hub card is also responsible for reset synchronization and for centralized arbitration. Centralized arbitration is performed by a programmable array logic (PAL™) device 115 of the master ISA hub card. Although a PAL™ device is included in every ISA hub card, only the PAL™ device in a master ISA hub card is enabled.

The master ISA hub card's PAL™ device 115 (also called master arbitration PAL™ device) performs centralized arbitration by monitoring request signals on a number of request lines of cable 120. Each request signal originates from one of ISA hub cards 101-103 as follows. The PAL™ device of a ISA hub card that receives a packet from an ethernet node drives a request signal active and waits for an acknowledge signal to go active before transmitting the received packet to other ISA hub cards e.g. on cable 120.

Master arbitration PAL™ device 115 generates a collision signal that is transmitted on a collision line to all ISA hub cards 101-103 when more than one request line is active simultaneously. If only one request line has an active request signal, master arbitration PAL™ device 115 generates an acknowledge signal that is also transmitted to all ISA hub cards 101-103. Only the ISA hub card that generated the active request signal responds to the active acknowledge signal and transmits a packet e.g. on cable 120. All other ISA hub cards wait to receive the packet in response to the active acknowledge signal.

The principles of centralized arbitration are explained in detail in the prior art, for example, in section 5.5.2 at page 5-9 of "IEEE 802.3 Repeater, Technical Manual" that was incorporated by reference above. Equations for implementing such a master arbitration PAL™ device are also well known, for example, as listed in section 5.5.4 at page 5-10 of "IEEE 802.3 Repeater, Technical Manual" (above).

Centralized arbitration has several drawbacks. For example, centralized arbitration requires that a slave ISA hub card 101-103 wait for the time required for a request signal to travel the expansion cable distance (e.g. distance D in FIG. 1A) between the slave ISA hub card and master ISA hub card and the time required for an acknowledge signal to travel back the expansion cable distance from the master ISA hub card, which wait imposes a limitation on the speed of the network (such as an ethernet). In one prior art ethernet, a slave ISA hub card waits for a travel time of 100 nanoseconds, which wait limits the prior art ethernet to a speed of 10 megahertz. Also, a malfunction in the master arbitration PAL™ device can cause unpredicted and improper operation of the aggregate hub.

SUMMARY OF THE INVENTION

A novel distributed arbitration apparatus in accordance with this invention includes a plurality of electrical devices (such as Industry Standard Architecture (ISA) hub cards), that independently determine availability of a shared resource (such as a bus that interconnects the devices), by use of an arbiter in each of the electrical devices. One of the electrical devices, called master device, generates a clock signal that is supplied to all the other electrical devices, called slave devices. All the electrical devices synchronize to the clock signal i.e. generate signals, such as a request signal, synchronously and sample signals, such as a grant signal, synchronously.

When a circuit in an electrical device needs to use the shared resource, the circuit drives a request signal active to a local arbiter (for example, in the form of a programmable logic device, such as a Programmable Array Logic (PAL™) device) included in that electrical device. The local arbiter in turn transmits the request signal to all other arbiters of devices that use the shared resource. Therefore, each arbiter receives and monitors request signals from all other arbiters.

If only one request signal is active at a given time, all arbiters receive the active request signal and each arbiter drives an acknowledge signal active to the respective local circuit. The circuit that requested the bus then uses the bus.

If more than one request signal is active simultaneously, each arbiter determines the occurrence of a collision, and drives a collision signal active to the respective local circuit that then implements a backoff algorithm, such as, but not limited to an exponent binary algorithm.

The novel distributed arbitration method and apparatus described above has several advantages. Distributed arbitration is faster because a circuit receives an acknowledge signal or a collision signal from an arbiter that is locally connected to the circuit rather than from a remote arbiter, as was necessary in the prior art. So the length of the bus connecting the arbiters is a less critical dimension for implementing distributed arbitration than for centralized arbitration. A longer bus made possible by the current invention allows a larger number of arbiters to be connected together to form a larger aggregate hub than possible in the prior art.

Also, distributed arbitration described above requires a smaller number of conductors in the bus as compared to the prior art because two signals namely acknowledge and collision signals no longer need to be transmitted over the bus, and instead only one signal, namely the request signal is transmitted on the bus. Distributed arbitration also provides better signal integrity and better scalability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates three prior art ISA hub cards that are supported by a system board.

FIG. 1B illustrates a prior art cable for interconnecting the ISA hub cards of FIG. 1A.

FIG. 1C illustrates in block diagram the interconnection of the ISA hub cards of FIG. 1A.

FIG. 2A illustrates use of the distributed algorithm of this invention by PAL™ devices in two ISA hub cards in one embodiment.

FIG. 2B illustrates the signals between PAL™ devices in four ISA hub cards in one embodiment.

FIG. 3A illustrates a system board for interconnecting four ISA hub cards in another embodiment of this invention.

FIG. 3B illustrates an ISA hub card for use with the system board of FIG. 3A.

DETAILED DESCRIPTION

One embodiment of this invention includes a plurality of electrical devices (described below), each of which has an arbiter (also described below) that independently determines availability of a shared resource, such as a bus included in identity cable 200 that interconnects the electrical devices.

Identity cable (also called simply "cable") 200 includes a first cable connector 201 located at a left end of cable 200, a second cable connector 202 located next to first cable connector 201, a third cable connector 203 located next to second cable connector 202 and a fourth cable connector 204 located at a right end of cable 200.

Each of cable connectors 201-204 includes a group of terminals 211-219, 221-229, 231-239 and 241-249 respectively. Cable 200 also includes a sheath 250 that supports and insulates electrical conductors, e.g. conductor 250A that interconnect various terminals between connectors of the identity cable. In two alternative embodiments, sheath 250 is a flexible ribbon or a rigid brace.

Connectors 201-204 are suitable for coupling to terminals of an electrical device, such as one of ISA hub cards 251 and 252. Although only two ISA hub cards 251 and 252 are illustrated in FIG. 2A, a total of four electrical devices can be interconnected by cable 200, as illustrated in FIG. 2B.

In the embodiment illustrated in FIG. 2A, two terminals of each cable connector, such as terminals 211 and 219 of cable connector 201, are "identity terminals" that can uniquely identify an electrical device attached to cable 200, such as one of ISA hub cards 251-252. The attached electrical device, e.g. ISA hub card 251 couples identity terminals 211 and 219 through pullup resistors 253 and 254 respectively to a voltage source, at for example, the Vcc reference (FIG. 2A). In one specific embodiment, each pullup resistor has 1000 Ω resistance. In the embodiment of FIG. 2A, each cable connector also includes two terminals, such as terminals 212 and 218 of cable connector 201, that are "ground terminals" coupled by the attached electrical device, e.g. ISA hub card 251, to a ground reference source.

Cable 200 provides a unique identity to two or more attached electrical devices as follows. Each cable connector can include one or more conductive links such as wires that couple an identity terminal to a ground terminal, for example, to indicate logic state "1". Alternatively an identity terminal is left unconnected inside cable 200, for example, to indicate logic state "0".

In the embodiment of FIG. 2A, identity terminals 211 and 219 are left unconnected so that cable connector 201 can indicate the identity "00". Identity terminal 221 is left unconnected while identity terminal 229 is connected to ground terminal 228 by a short conductive link 255, so that cable connector 202 can indicate the identity "01". Identity terminals 231 and 239 are connected by conductive links 256 and 257 to ground terminals 232 and 238 respectively so that cable connector 203 can indicate the identity "11". Finally, identity terminal 241 is connected by conductive link 258 to ground terminal 242, while identity terminal 249 is left unconnected so that cable connector 204 can indicate the identity "10".

The rest of the terminals in cable 200 that carry conventional signals are connected with straight through wiring in the conventional manner, for example, wire 250A connects terminals 213, 223, 233 and 243. For clarity, not all wires of cable 200 are shown in FIGS. 2A and 2B.

Such an identity cable 200 eliminates the hub card's jumpers or dip switches that were needed in prior art ISA hub cards 101-103 (above) to indicate identity. Cable 200 can provide unique identities to each attached electrical device, irrespective of the specific position of the electrical device with respect to cable 200, even if the electrical devices are all identical to each other.

In one embodiment, identity conductors are located at the outer edges (FIG. 2A) of identity cable 200, to assure stable impedance characteristics for identity cable 200.

If there is a "master-slave" relationship wherein one electrical device is a master and all other electrical devices are slaves, then one electrical device must be connected to the specific connector that indicates master identity, while other electrical devices can be connected to any other connector of cable 200. For example, when cable 200 is used with ISA hub cards, one ISA hub card must be connected to the cable connector that indicates the identity "00", such as cable connector 201, so that the aggregate ISA hub formed by cable 200 has one (and only one) master. Use of the specific identity "00" for a master has the advantage that in an ISA hub with only one ISA hub card, e.g. card 251, the connector, e.g. connector 259, can be left unconnected and yet the ISA hub has an ISA hub card, e.g. card 251, with master identity "00".

The function of sensing identity in an attached electrical device is performed by an arbiter. An arbiter can be implemented in a programmable logic device (PLD) such as one of PAL™ devices 261 and 262 of ISA hub cards 251 and 252 respectively. Each one of PAL™ devices 261 and 262 is similar to PAL™ devices of a prior art ISA hub card. Equations for a PAL™ device in one embodiment of an ISA hub card of this invention are listed in Table A, are in Data I/O's Abel language, and can be used with Data I/O's Unisite programmer to program, for example, PALCE20V8-25, also available from AMD™, Inc. to form an arbiter.

In this embodiment, all portions of the ISA hub card that are not disclosed herein are identical to those in a prior art ISA hub card, as described in, for example, ISA-HUB™ User's Manual referenced above.

Each one of ISA hub cards 251 and 252 that is attached to cable 200 has a number of identity lines, such as identity lines 263 and 264, that are coupled to identity terminals, such as identity terminals 211 and 219, when the electrical device is attached to an identity connector. Each one of PAL™ devices 261 and 262 senses the voltage level (and hence the logic state) of the signals on the identity lines to determine the identity of ISA hub cards 251 and 252 respectively. When connectors 201 and 204 are coupled to ISA hub cards 251 and 252 respectively, ISA hub card 251 senses identity "00" and acts as master while the other ISA hub card 252 senses the identity "10" and acts as a slave. As noted above, the connections of connectors 201 and 204 and cards 251 and 252 can be reversed without any loss of functionality.

In one embodiment, PAL™ devices 261 and 262 implement a novel distributed arbitration method in which collision and acknowledge signals are generated by each PAL™ device and supplied to a repeater circuit that is directly connected to the PAL™ device within a ISA hub card. When a repeater circuit, such as repeater circuit 267 needs to transmit a packet on cable 200, the repeater circuit drives a request signal active on one of the wires of cable 200. Each PAL™ device, such as PAL™ device 261 of ISA hub card 251 monitors request signals on request lines REQ1-REQ4 (FIG. 2B) from all repeater circuits (e.g. repeater circuits 267-270) that are connected by cable 200. Each PAL™ device (e.g. PAL™ device 26) receives a request signal from a local repeater circuit (e.g. repeater circuit 267) on for example a local request terminal 261A coupled to a local request line (e.g. line REQ1) of the ISA hub card (e.g. card 251). Each PAL™ device (e.g. PAL™ device 261) also receives request signals from remote repeater circuits (e.g. circuit 268) on remote request terminals 261B-261D couplable to wires (not shown) of cable 200.

Each one of PAL™ devices 261, 262, 271 and 272 determines the occurrence of a collision, if more than one request signal, e.g. on request lines REQ1-REQ4, is active simultaneously, and then drives a collision signal active, e.g. on each of collision terminals 261E, 271E, 272E and 262E coupled to collision lines COL1-COL4 of the respective local repeater circuit e.g. each of repeater circuits 267-270. If only one request signal is active at a given time, all PAL™ devices receive the active request signal on e.g. request line REQ1, and each of PAL™ devices 261, 262, 271 and 272 drives an acknowledge signal active e.g. on acknowledge terminals 261F, 271F, 272F and 262F coupled to acknowledge line ACK1 of the respective local repeater circuit, e.g. repeater circuits 267-270.

The novel distributed arbitration method described above has several advantages. The distributed arbitration scheme is faster because an electrical device's circuit, such as repeater circuit 267, receives an acknowledge signal on acknowledge line ACK1 or a collision signal on collision line COL1 from an arbiter that is locally connected to the repeater circuit, such as PAL™ device 265, rather than from a remote arbiter, as was necessary in the prior art. Therefore, the expansion cable distance of cable 200 is a less critical dimension for implementing distributed arbitration than for centralized arbitration. For example, an aggregate ISA hub formed of ISA hub cards 251-252 and 273-274 that are connected by an identity cable of the same length as the prior art expansion cable distance can run at twice the prior art speed. Alternatively, an aggregate ISA hub running at the same speed as a prior art speed can be formed of twice as many ISA hub cards as in the prior art because the expansion cable distance can be twice the prior art expansion cable distance. The doubled speed or the doubled distance of this invention are possible because a repeater circuit, e.g. one of circuits 267-270 is only limited by the travel time of a single expansion cable distance (for the request signal).

The distributed arbitration method described above requires a smaller number of straight-through wires in cable 200, as compared to the prior art because the acknowledge signal and the collision signal are no longer transmitted over cable 200, although the request signal from a repeater circuit of a master ISA hub card is transmitted on cable 200. The master repeater circuit's request signal, e.g. on request line REQ1 allows slave ISA hub cards, e.g. cards 252, 273 and 274 to arbitrate in parallel with the master ISA hub card, e.g. card 251. For example, in the embodiment illustrated in FIG. 2A, identity cable 200 has four request lines 213-217, as compared to prior art cable 120 that has only three request lines 133, 134 and 135 but also has two lines 132 and 133 for acknowledge and collision signals. Distributed arbitration also provides better signal integrity.

Although the novel distributed arbitration method has been described above with an identity cable specifying the identity of an electrical device, other identity mechanisms can be used, including for example, user configurable elements such as jumpers, dip-switches and programmable fuses. For example, a PAL™ device 261 can include an identity terminal 261G (FIG. 2B) coupled on card 251 to a jumper 281 that can be set by a user to indicate different identities of card 251. Programmable fuses can be, for example, part of a field programmable gate array included in an integrated circuit that contains an arbiter and a repeater circuit.

Also, although a novel distributed arbitration method is described above for use in ISA hub cards connected by an identity cable, the novel identity apparatus can be used with other arbitration methods. For example, a prior art centralized arbitration method can be implemented in ISA hub cards that sense identities from an identity cable. Moreover, the novel distributed arbitration method can be used in ISA hub cards with prior art identity mechanisms, such as manually configured jumpers or dip-switches.

Although specific connections between ISA hub cards 251-252 and cable 200 are illustrated in FIG. 2A, other connections are also possible. For example, connector 259 of ISA hub card 251 can be connected to fourth cable connector 204 while connector 256 of ISA hub card 252 is connected to first cable connector 201. Since ISA hub cards 251 and 252 are identical to each other, such a connection, that is reverse of the connection shown in FIG. 2A, still results in the same functionality. Similarly connecting first connector 201 to ISA hub card 251 and second connector 202 to ISA hub card 252 results in the same functionality.

An even number of identity terminals (e.g. two identity terminals) provides symmetry to cable 200 that permits reversibility (e.g. cable 200 providing identities "01", "00", "11" and "10" in a first orientation with respect to four electrical devices can be connected in a second orientation spatially reversed with respect to the first orientation to provide identities "10", "11", "00" and "01"), without loss of uniqueness of identity. When multiple hub cards are interconnected by cable 200, the specific identity of each hub card is irrelevant as long as there is only one master hub card and all other hub cards have identities unique with respect to each other.

Numerous variations in identity order can cover the entire permutation of possible identities indicated by an identity cable. For example, in a variation of the embodiment of cable 200 described above, identity terminals 221, 229 and 211 can be left unconnected while terminal 219 is connected to ground terminal 218, so that cable connector 202 indicates the identity "00" and cable connector 201 indicates the identity "01." Other identities "10" and "11" can also be indicated by cable connector 202 by appropriate coupling of identity terminals to the ground reference source. The range of possible identities can be expanded by increasing the number of identity terminals in an identity apparatus, for example from 2 identity terminals (four identities) to 4 identity terminals (sixteen identities). Other numbers of identity terminals are also possible.

Although wires 241 and 242 are used in one embodiment as conductive links to couple the identity terminals and the ground terminals, other electrical couplings can also be used, and an identity apparatus can have a form other than a cable. In one embodiment, an inflexible structure, such as system board 300 (FIG. 3A) rigidly supports a number of identity connectors 301-304 that are similar to identity connectors 201-204 (FIG. 2A) except as described below. Connectors 301-304 (FIG. 3A) do not have ground terminals, such as ground terminals 212 and 218 (FIG. 2A). Instead, terminals 312 and 318 are connected straight through by traces (not shown) in system board 300 to terminals 322, 332 and 342 and to 328, 338 and 348 respectively and are used for signal transmission between ISA hub boards (not shown) attached to identity connectors 301-304. Identity terminals, such as terminals 311 and 319 (FIG. 3A) of connector 301 are connected inside system board 300 to the ground reference source or pulled up through pull-up resistors, e.g. resistors 353-354 to the Vcc reference, as necessary to indicate an identity.

For use with system board 300, an attached electrical device e.g. ISA hub card 360 (FIG. 3B) does not provide connections to a source of the ground reference or of the Vcc reference as described above for ISA hub cards 251 and 252. However, each attached electrical device e.g. ISA hub card 360 senses the voltage on the identity lines e.g. lines 363 and 364, in the manner described above e.g. for lines 263 and 264 of ISA hub card 251.

An identity apparatus e.g. cable 200 or system board 300 that is external to an electrical device to be identified, as described above, has several advantages. Each of the electrical devices is identical to every other electrical device, resulting in elimination of jumpers (or dip switches) and thus reducing the electrical device's fabrication cost. Moreover, the cost of assembling an aggregate ISA hub that includes a number of ISA hub cards is reduced, due to elimination of the error-prone, laborious step of setting jumpers or dip switches. Use of identity cable 200 or system board 300 eases assembly of such an aggregate ISA hub.

Numerous other advantages of an identity apparatus will be obvious to those skilled in the art of "plug and play", based on the enclosed disclosure. For example, as minimal forethought is needed for connecting an identity apparatus to an electrical device, this invention results in time saving during assembly. Also, reversibility of the identity apparatus results in idiot-proof assembly.

Numerous modifications and adaptations of the invention will be obvious in view of the enclosed disclosure. For example, a system board can contain only traces that connect identity terminals to a ground reference source (as described in reference to FIG. 3A), while the identity terminals are coupled by pull-up resistors in the attached electrical devices to a voltage source (as described in reference to FIG. 2A).

Although negative logic states are illustrated in the above description, wherein the Vcc reference indicates logic state "0" and the ground reference indicates logic state "1", positive logic states with logic states reverse of the negative logic states can also be used.

Although PAL™ device embodiment has been described above, an arbiter of this invention can be embodied in a part of an integrated circuit, and furthermore, a repeater circuit can be implemented as another part of the same integrated circuit (or a different integrated circuit).

Although in one embodiment, distributed arbitration and arbiters are used to share the use of a bus connecting a plurality of electrical devices, distributed arbitration and arbiters can be used to share other resources that are always available (e.g. a spinning disk drive) and used by the requesting electrical device.

Various modifications and adaptations of the embodiments described above are encompassed by the attached claims.

                                      TABLE A                                      __________________________________________________________________________     APPENDIX A                                                                     __________________________________________________________________________     MODULE ISAHUB1                                                                 TITLE  `ISAHUB1.ABL, Rev. 1.0, 10/02/94`                                       __________________________________________________________________________     DECLARATIONS                                                                   ISAHUB1                                                                               device `MACH110A`;                                                      "IMR+ Signals"                                                                 SO     PIN 30;                                                                 STR    PIN 35;                                                                 |REQ   PIN 32;                                                                 X1     PIN 14 istype `Com,Buffer`;                                             TEST   PIN 9  istype `Com,Buffer`;                                             JAM    PIN 21 istype `Com,Buffer`;                                             DAT    PIN 20 istype `Com,Buffer`;                                             |ACK   PIN 15 istype `Com,Invert`;                                             |COL   PIN 6  istype `Com,Invert`;                                             |RST   PIN 2  istype `Reg,Invert`;                                             SI     PIN 5  istype `Reg,Buffer`;                                             "Expansion Interface Signals"                                                  |S1    PIN 10;                                                                 |S0    PIN 11;                                                                 XCLK   PIN 8  istype `Com,Buffer`;                                             BJAM   PIN 18 istype `Com,Buffer`;                                             BDAT   PIN 19 istype `Com,Buffer`;                                             |REQ3  PIN 24 istype `Com,Invert`;                                             |REQ2  PIN 27 istype `Com,Invert`;                                             |REQ1  PIN 3  istype `Com,Invert`;                                             |REQ0  PIN 4  istype `Com,Invert`;                                             |INIT  PIN 7  istype `Reg,Invert`;                                             "LED Interface Signals"                                                        TCLK   PIN 16 istype `Reg,Buffer`;                                             |LED5  PIN 38 istype `Reg,Invert`;                                             |LED4  PIN 39 istype `Reg,Invert`;                                             |LED3  PIN 40 istype `Reg,Invert`;                                             |LED2  PIN 41 istype `Reg,Invert`;                                             |LED1  PIN 42 istype `Reg,Invert`;                                             |LED0  PIN 43 istype `Reg,Invert`;                                             TP7    NODE   istype `Reg,Buffer`;                                             TP6    NODE   istype `Reg,Buffer`;                                             TP5    NODE   istype `Reg,Buffer`;                                             TP4    NODE   istype `Reg,Buffer`;                                             TP3    NODE   istype `Reg,Buffer`;                                             TP2    NODE   istype `Reg,Buffer`;                                             TP1    NODE   istype `Reg,Buffer`;                                             TP0    NODE   istype `Reg,Buffer`;                                             "Misc. Signals"                                                                CLK        PIN 13;                                                             OSC20MHZ   PIN 33;                                                             RESETDRV   PIN 17;                                                             INITX      NODE;                                                               "Vectors"                                                                      TP70           =  TP7. .TP0!                                                   TP71           =  TP7. .TP1!                                                   TP60           =  TP6. .TP0!;                                                  TP50           =  TP5. .TP0!;                                                  LED            =  LED5. .LED0!;                                                REQ30          =  REQ3. .REQ0!;                                                EQUATIONS                                                                      "IMR+ Interface Logic"                                                         X1             = XCLK;                                                         RST            := INIT;                                                        TEST           = RST.d;                                                        SI             := RST.d # SI.d & |TP3;                                         COL            = |ACK.fb & (REQ3 # REQ2 # REQ1                                                # REQ0);                                                        ACK            = REQ3 & |REQ2 & |REQ1 & |REQ0                                                 # |REQ3 & REQ2 & |REQ1 & |REQ0                                                 # |REQ3 & |REQ2 & REQ1 & |REQ0                                                 # |REQ3 & |REQ2 & |REQ1 & REQ0;                                 JAM            = BJAM;                                                         DAT            = BDAT;                                                         "Expansion Interface Logic"                                                    XCLK           = OSC20MHZ;                                                     INITX          := RESETDRV;                                                    INIT           := INITX;                                                       REQ30          = REQ;                                                          BJAM           = JAM;                                                          BDAT           = DAT;                                                          "LED Interface Logic"                                                          TCLK           := |RST.d & |TCLK.d;                                            TP7            := |RST.d & (SI.d # |TCLK.d & SO #                              TCLK.d & TP7);                                                                 TP60           := |RST.d & (|TCLK.d & TP71 # TCLK.d &                          TP60)                                                                          LED            := |RST.d & (|TCLK.d & STR & TP50 #                                            |(|TCLK.d & STR) & LED.d);                                      "TRISTATES"                                                                    REQ3.oe        = |S1 & |SO;                                                    REQ2.oe        = |S1 & SO;                                                     REQ1.oe        = S1 & |SO;                                                     REQ0.oe        = S1 & SO;                                                      INIT.oe        = S1 & SO;                                                      XCLK.oe        = S1 & SO;                                                      BJAM.oe        = ACK.fb & REQ;                                                 BDAT.oe        = ACK.fb & REQ;                                                 JAM.oe         = |REQ;                                                         DAT.oe         = |REQ;                                                         "CLOCK SETTINGS"                                                               RST.clk        = CLK;                                                          SI.clk         = CLK;                                                          INITX.clk      = CLK;                                                          INIT.clk       = CLK;                                                          TCLK.clk       = CLK;                                                          TP70.clk       = CLK;                                                          LED.clk        = CLK;                                                          END                                                                            __________________________________________________________________________ 

I claim:
 1. A system comprising a plurality of electrical devices coupled to one another through a bus, each electrical device comprising:a circuit having a local request line and an acknowledge line; an arbiter comprising (i) a local request terminal coupled to said local request line, (ii) an acknowledge terminal coupled to said acknowledge line, (iii) a plurality of remote request terminals, each remote request terminal being coupled through said bus to a local request line of a circuit of an electrical device other than said electrical device and (iv) an identity terminal coupled to a second apparatus terminal of an identity apparatus, said second apparatus terminal being coupled inside said identity apparatus to a first apparatus terminal, such that presence of coupling between said first apparatus terminal and said second apparatus terminal indicates an identity to said arbiter when said arbiter is coupled to said identity apparatus; wherein said arbiter drives a signal active on said acknowledge terminal in response to an active signal on said local request terminal and in absence of an active signal on any of said remote request terminals; wherein said first apparatus terminal is coupled through a pull-up resistor to a voltage source in said electrical device, when said electrical device is coupled to said identity apparatus, said voltage source supplying a signal of an approximately fixed predetermined voltage level indicative of a logic state in said electrical device.
 2. The system of claim 1 wherein said predetermined voltage level is the Vcc reference.
 3. The system of claim 1 wherein said predetermined voltage level is the ground reference.
 4. The system of claim 1, wherein said apparatus terminals and a plurality of terminals of said bus are included in a single connector.
 5. The system of claim 4, wherein said connector is one of a plurality of connectors of a system board included in said system.
 6. The system of claim 4, wherein said connector is one of a plurality of connectors of a cable included in said system.
 7. The system of claim 1, wherein:said circuit includes a repeater circuit having a collision line; and said arbiter further comprises a collision terminal coupled to said collision line, and in response to said active signal on said local request terminal and an active signal on any remote request terminal said arbiter drives a signal active on said collision terminal.
 8. A system comprising a plurality of electrical devices coupled to one another through a bus each electrical device comprising:a first circuit having a first line and a second line; a second circuit comprising (i) a first terminal coupled to said first line, (ii) a second terminal coupled to said second line, (iii) a plurality of third terminals, each third terminal being coupled through said bus to a first line of a first circuit of an electrical device other than said electrical device, and (iv) an identity terminal coupled to a second apparatus terminal of an identity apparatus, said second apparatus terminal being coupled inside said identity apparatus to a first apparatus terminal, such that presence of coupling between said first apparatus terminal and said second apparatus terminal indicates an identity to said second circuit when said first apparatus terminal and said second apparatus;wherein: said second circuit drives a signal active on said second terminal in response to an active signal on said first terminal and in absence of an active signal on any of said third terminals; and each of said electrical devices comprises a plurality of identity terminals, each unique combination of signals on said identity terminals indicating a unique identity to an electrical device.
 9. The system of claim 8, wherein said apparatus terminals and a plurality of terminals of said bus are included in a single connector.
 10. The system of claim 9, wherein said connector is one of a plurality of connectors of a system board included in said system.
 11. The system of claim 9, wherein said connector is one of a plurality of connectors of a cable included in said system.
 12. The system of claim 8, whereinsaid first circuit includes a repeater circuit having a third line; and said second circuit further comprises a fourth terminal coupled to said third line, and in response to said active signal on said first terminal and an active signal on any third terminal said second circuit drives a signal active on said fourth terminal.
 13. The system of claim 8 wherein said first circuit is included in an integrated circuit.
 14. The system of claim 8 wherein said second circuit is included in a programmable logic device.
 15. The system of claim 8 wherein said first circuit and said second circuit are both included in a single integrated circuit.
 16. The system of claim 8 further comprising a user configurable element, wherein said identity terminal is coupled to said user configurable element.
 17. The system of claim 16 wherein said user configurable element is a jumper.
 18. The system of claim 8 wherein said first apparatus terminal is coupled to a voltage source in said electrical device when said electrical device is coupled to said identity apparatus.
 19. A system comprising a plurality of electrical devices coupled to one another through a bus each electrical device comprising:a circuit having a local request line and an acknowledge line: and an arbiter comprising (i) a local request terminal coupled to said local request line, (ii) an acknowledge terminal coupled to said acknowledge line, and (iii) a plurality of remote request terminals, each remote request terminal being coupled through said bus to a local request line of a circuit of an electrical device other than said electrical device, wherein said arbiter drives a signal active on said acknowledge terminal in response to an active signal on said local request terminal and in absence of an active signal on any of said remote request terminals; the system further comprising:an identity apparatus comprising a housing and a connector supported by said housing, said connector comprising a plurality of apparatus terminals including said first apparatus terminal and said second apparatus terminal, wherein when an electrical device is coupled to said connector:said first apparatus terminal of said plurality of apparatus terminals is coupled to a first device terminal in said electrical device, said first apparatus terminal carrying a first signal supplied at said first device terminal, said first signal having an approximately fixed predetermined first voltage level; and said second apparatus terminal of said plurality of apparatus terminals is coupled to a second device terminal in said electrical device, said second apparatus terminal carrying a second signal supplied at said second device terminal, said second signal having a predetermined fixed second voltage level; wherein said first apparatus terminal is coupled inside said identity apparatus to said second apparatus terminal, said second signal appears on said first device terminal and thereby indicates a second identity to said electrical device different from a first identity indicated by appearance of said first signal on said first device terminal.
 20. The system of claim 19 wherein each circuit is a repeater circuit of a hub adapter card.
 21. The system of claim 19 wherein said identity apparatus comprises a plurality of connectors, more than one connector being coupled to an electrical device such that each coupled connector indicates an identity different from the identity indicated by any other coupled connector.
 22. The system of claim 19:wherein said identity apparatus comprises a plurality of connectors, each connector having apparatus terminals arranged symmetric with respect to each other; wherein said identity apparatus is spatially located with respect to a location of said plurality of electrical devices such that said connectors indicate a plurality of identities in a first sequence, each identity in said first sequence being different from every other identity in said first sequence; and further wherein said plurality of connectors indicate a second sequence of identities, said second sequence being in the reverse order of said first sequence when said identity apparatus is spatially reversed with respect to said location of said plurality of electrical devices.
 23. The system of claim 19, wherein said apparatus terminals and a plurality of terminals of said bus are included in a single connector.
 24. The system of claim 23, wherein said connector is one of a plurality of connectors of a system board included in said system.
 25. The system of claim 23, wherein said connector is one of a plurality of connectors of a cable included in said system.
 26. An electrical device comprising:a first circuit having a plurality of lines; a second circuit having a plurality of terminals including a group of device terminals and a group of internal terminals, each internal terminal being coupled to a line of said first circuit, each device terminal being coupled to a bus external to said electrical device, wherein said second circuit drives a signal active on an internal terminal in response to an active signal on another internal terminal and in absence of an active signal on any of said device terminals; and an identity cable having a left end and a right end, said cable comprising:a plurality of cable connectors a left cable connector of said plurality of cable connectors being located at said left end, a right cable connector of said plurality of cable connectors being located at said right end, each of said cable connectors comprising a plurality of cable terminals, at least one of said cable terminals being predetermined to be coupled to a device terminal in each of said electrical devices, said device terminal supplying a first signal at a predetermined first voltage level; a plurality of electrical conductors comprising said bus, each electrical conductor being flexible, each electrical conductor being insulated from another electrical conductor, at least one electrical conductor being connected to said predetermined cable terminal each of said cable connectors;wherein: at least two cable terminals in one of said plurality of cable connectors are coupled to each other such that appearance of said first signal on one of said two cable terminals indicates an identity of one of said electrical devices, and said cable terminals in each connector are arranged symmetric with respect to each other; said identity cable is spatially located and connected to said plurality of electrical devices in a first orientation such that said cable connectors indicate a plurality of identities in a first sequence, each identity in said first sequence being different from every other identity in said first sequence; and said plurality of cable connectors indicate a second sequence of identities, said second sequence being in the reverse order of said first sequence when said identity cable is connected in a second orientation spatially reversed with respect to said first orientation.
 27. The system of claim 26, wherein:said first circuit includes a repeater circuit coupled to said second circuit; and in response to said active signal on said another internal terminal and an active signal on any device terminal said second circuit drives a signal active to said repeater circuit.
 28. The electrical device of claim 26 wherein said first circuit is included in an integrated circuit.
 29. The electrical device of claim 26 wherein said second circuit is included in a programmable logic device.
 30. The electrical device of claim 26 wherein said first circuit and said second circuit are both included in a single integrated circuit.
 31. The electrical device of claim 26 further comprising a user configurable element, wherein one of said device terminals is coupled to said user configurable element.
 32. The electrical device of claim 31 wherein said user configurable element is a jumper.
 33. The electrical device of claim 31 wherein said user configurable element is coupled to a voltage source in said electrical device.
 34. A system comprising:a plurality of electrical devices, each electrical device comprising a plurality of device terminals, at least one of said device terminals being an identity terminal, wherein a signal on one or more identity terminals indicates a unique identity to said electrical device; and an identity apparatus comprising a housing and a connector supported by said housing, said connector comprising a plurality of apparatus terminals, wherein when one of said electrical devices is coupled to said connector: a first apparatus terminal of said plurality of apparatus terminals is coupled to a first device terminal in said electrical device, said first apparatus terminal carrying a first signal supplied at said first device terminal, said first signal having an approximately fixed predetermined first voltage level; and a second apparatus terminal of said plurality of apparatus terminals is coupled to a second device terminal in said electrical device, said second device terminal being an identity terminal, wherein said first apparatus terminal is coupled inside said identity apparatus to said second apparatus terminal, and appearance of said first signal on said identity terminal indicates an identity of said electrical device different from an identity of said electrical device on appearance of a second signal on said identity terminal.
 35. The system of claim 34 wherein each circuit is a repeater circuit of a hub adapter card.
 36. The system of claim 34 wherein said second apparatus terminal carries a ground reference signal supplied at said second device terminal in the absence of coupling inside said identity apparatus between said first apparatus terminal and said second apparatus terminal.
 37. The system of claim 34 wherein said identity apparatus comprises a plurality of connectors, more than one connector being coupled to an electrical device such that each coupled connector indicates an identity different from the identity indicated by any other coupled connector.
 38. The system of claim 34:wherein said identity apparatus comprises a plurality of connectors, each connector having apparatus terminals arranged symmetric with respect to each other;wherein said identity apparatus is spatially located with respect to a location of said plurality of electrical devices such that said connectors indicate a plurality of identities in a first sequence, each identity in said first sequence being different from every other identity in said first sequence; and further wherein said plurality of connectors indicate a second sequence of identities, said second sequence being in the reverse order of said first sequence when said identity apparatus is spatially reversed with respect to said location of said plurality of electrical devices.
 39. The system of claim 34 wherein:said housing is a flexible tubular sheath and said plurality of apparatus terminals are connected to wires flexibly enclosed by said sheath to form a cable; and said cable has a plurality of connectors identical to said connector.
 40. The system of claim 34 wherein;said housing is an inflexible system board and at least one apparatus terminal of said connector is connected to a voltage source in said system board; and said system board has a plurality of connectors identical to said connector.
 41. The system of claim 34 wherein said first apparatus terminal is coupled through a pull-up resistor to a voltage source in said electrical device.
 42. The system of claim 34 wherein said first apparatus terminal is coupled through a pull-up resistor to a voltage source in said identity apparatus.
 43. The system of claim 34 further comprising a conductive link, said conductive link forming said coupling between said first apparatus terminal and said second apparatus terminal.
 44. The system of claim 34 wherein said signal has an approximately fixed predetermined voltage level indicative of a logic state in said electrical device.
 45. The system of claim 44 wherein said predetermined voltage level is the Vcc reference.
 46. The system of claim 34 comprising a plurality of first apparatus terminals, wherein each unique combination of signals on said first apparatus terminals indicates a unique identity of said electrical device.
 47. A method for distributed arbitration of a resource shared by a plurality of electrical devices, each electrical device comprising a circuit having a local request line and an acknowledge line, said method comprising:providing an arbiter in each of said electrical devices, said arbiter comprising a local request terminal, an acknowledge terminal and a plurality of remote request terminals, said providing step comprising coupling said local request terminal to said local request line and coupling said acknowledge line to said acknowledge terminal coupled to said acknowledge line; causing said arbiter to drive a signal active on an acknowledge terminal in response to an active signal on said local request terminal and in absence of an active signal on any of said remote request terminals, wherein each electrical device comprises a plurality of device terminals, at least one of said device terminals being an identity terminal, wherein a signal .right brkt-top.. one or more identity terminals is capable of indicating a unique identity to said electrical device; supporting a first apparatus terminal and a second apparatus terminal in a connector of an identity apparatus such that said first apparatus terminal is coupled to a first predetermined device terminal in a first electrical device of said plurality of electrical devices and said second right terminal is coupled to a second predetermined device terminal in said first electrical device when said first electrical device is coupled to said connector; and coupling inside said connector said second apparatus terminal to said first apparatus terminal so that a signal supplied by said first electrical device to said first apparatus terminal can appear on said second apparatus terminal to indicate an identity of said first electrical device.
 48. A method comprising the steps of claim 47 followed by:coupling said identity apparatus to said first electrical device; and supplying said signal to said first apparatus terminal.
 49. The method of claim 47 wherein the arbiter has a collision terminal, the method further comprising:causing said arbiter to drive a signal active on said collision terminal in response to said active signal on said local request terminal and presence of an active signal on any remote request terminal of said arbiter.
 50. The method of claim 49 further comprising:implementing a backoff algorithm in response to said active signal on said collision terminal.
 51. The method of claim 47 further comprising:supplying a clock signal to each of said electrical devices.
 52. The method of claim 51 further comprising:using a user configurable element in each of said electrical devices to identify one of said electrical devices as a master electrical device, wherein said master electrical device supplies said clock signal.
 53. The method of claim 47 further comprising:using said central resource in response to said active signal on said acknowledge terminal. 